THIS invention relates to a method of and a system for monitoring electromagnetic interference.
Electromagnetic Interference (EMI) signals are emitted by most high voltage (HV) apparatus or equipment. These signals or emissions usually cover a wide bandwidth and are determined by, amongst other things, operating voltage, apparatus design and geometry, insulation class and condition. Additionally, depending on the method of acquisition, the associated spectra of these signals may be polluted by components from extraneous sources. Specifically, generator EMI spectra cover a bandwidth from very low frequencies up to about 1 GHz. The most important component of this band is from approximately 150 kHz to 250 MHz. The signals in this bandwidth typically approach true randomness and the discrepancy between peak (PK) and quasi-peak (QP) values can be as much as about 10 dB.
EMI spectra are captured by way of frequency domain acquisition equipment for example spectrum analysers or EMI test receivers (also known as RF receivers or frequency selective voltmeters). These types of devices are relatively expensive and proper representation of wideband chaotic EMI signals requires the implementation of very long scan times when using these frequency domain acquisition devices. The high cost of these devices and the long scan times required conspire against the practicality of widely deploying on-line real-time monitoring systems based on these acquisition options.
Another, alternate approach, is to capture a number of time domain representations of the electromagnetic emission and then in post processing, compile these time domain captures into a long pulse train that is statistically representative of the original emission. Generally, this statistically representative pulse train is then processed via a short-time discrete Fourier transform (STFT) to produce a spectrogram which is statistically representative of the original EMI emission. In particular, the STFT is a series of fast Fourier transforms (FFT), where the input to each FFT is a subset of the overall pulse train, much shorter in length than the entire pulse train. Successive FFTs process incrementally shifted subsets of the entire pulse train. The output is a set of FFTs, each representing the spectrum of the EMI emission at a different point in time. Such a set of FFTs is called a spectrogram. The FFTs of the spectrogram can be combined with one another to produce an output equivalent to an EMI receiver. The methodology of combination depends on the choice of detector (peak, quasipeak, etc.). Such systems are usually termed Time-Domain EMI (TDEMI) systems.
It is an object of the present invention to monitor EMI signals more conveniently and more cost effectively analysing time domain signals.